1. Field of the Invention
This invention relates to a fossil fuel fired, single-ended, self-recuperated radiant tube annulus system suitable for use in radiant tube heating applications, such as ferrous and nonferrous metal treatments, whereby, in addition to providing heat for transfer to the intended load, the products of combustion from the combustion of the fossil fuel may be used to preheat the oxidant used for the combustion of the fossil fuel.
2. Description of Prior Art
Radiant tubes have long been used in industrial heating applications for heating a variety of materials, such as steel or other solid materials in a furnace. Conventionally, radiant tube heaters have been powered by electrical heating elements or by fuel-fired burners. Electrically heated radiant tubes basically comprise heating elements within a tube which extend into a furnace or work zone. The elements radiate heat to the tube and the tube radiates heat to the work. In high temperature heating applications, electrically heated radiant tubes are preferred because the heating elements radiate uniform heat flux to the tube. However, the cost of electricity often dictates that fuel-fired burners be used in place of the electrical heating elements to provide products of combustion into a tube which, in turn, will radiate heat to the work.
In conventional radiant tube designs, the high-temperature combustion products are supplied into the radiant tube from one end thereof and, after having been used for heating, are then discharged from the other end thereof. It will be apparent that the discharged products of combustion still have a relatively high temperature. One system which has been developed to address this issue is a recuperative radiant tube burner system comprising an outer heat resistant radiant tube having a closed forward end in which is located an elongated recuperator tube which coats with the radiant tube to define an annular exhaust passage for the flow of hot gases produced by a burner assembly disposed within the recuperator tube as taught by U.S. Pat. No. 5,241,949 to Collier and U.S. Pat. No. 4,705,022, also to Collier. U.S. Pat. No. 5,016,610 to Meguro et al. teaches a radiant-type heater having inner and outer concentric tubes and a fuel supply tube disposed within the inner concentric tube. The end of the outer concentric tube opposite the fuel supply end of the heater is closed off as a result of which combustion products resulting from combustion of the fuel from the fuel supply tube in the inner concentric tube are exhausted through the annulus formed between the inner and outer concentric tubes. And, U.S. Pat. No. 4,401,099 to Collier teaches a single-ended recuperative radiant tube assembly having inner and outer recuperative tube assemblies positioned in a counterflow arrangement within a radiant tube assembly whereby hot exhaust gases emitted from the burner within the single-ended radiant tube assembly are directed through a flame tube to an annular exhaust chamber located between the outer recuperative tube and radiant tube assemblies. Ambient air flowing toward the burner in an air chamber between the inner and outer recuperator tube assemblies is heated by the exhaust gases in the annular exhaust chamber. See also U.S. Pat. No. 3,225,757 to Keller which teaches a radiant tube heater closed at one end having a central fuel conduit, an air conduit surrounding the fuel input conduit with orifices which enable air to pass through the walls of the air conduit, and a housing around the air conduit, whereby combustion occurs in the space between the housing and the air conduit; U.S. Pat. No. 4,062,343 to Spielman which teaches a tube firing burner having a tube, one end of which is supported within an opening in a heat-insulating wall and which is fired by a dual-fuel burner where, disposed within the end portion of the tube is a combustion sleeve which extends the flame of the burner inwardly beyond the wall, thereby preventing the insulated end portion of the tube from being burned out, and an abrupt restriction is formed at the discharge end of the combustion sleeve, creating turbulence in the air/fuel mixture so as to promote cleaner and more efficient burning; U.S. Pat. No. 3,946,719 to Bark et al. which teaches a radiant gas heater having an externally heat radiating shell, a central gas fuel tube, and an intermediate perforated air conducting tube from the inside of which air flows into a combustion chamber, all arranged coaxially; U.S. Pat. No. 4,531,904 to Sato et al. which teaches a radiant tube burner having a concentrically arranged outer air flow nozzle, fuel gas nozzles arranged in a circle within the outer air flow nozzle and an inner air flow nozzle; U.S. Pat. No. 4,140,482 to Simon which teaches a fuel-fired radiant-heating tube having an acoustic damper or muffler comprising a double-wall housing defining two elongated annular flow passages in indirect heat-exchange relationship through a thermally conductive inner wall whereby the combustion exhaust gases are conducted through one of these passages while combustion-sustaining gas, for example, air, is supplied through the other in counterflow to the exhaust gas, thereby preheating the air; U.S. Pat. No. 4,479,535 to Echigo et al. which teaches a radiant tube heated by high-temperature gases supplied from one end having a heat exchanger disposed in proximity to the gas outlet of the tube so as to recover thermal energy from the combustion gases before they are exhausted; and U.S. Pat. No. 5,224,542 to Hemsath which teaches a radiant tube heater having a gas-fired burner which fires heated products of combustion into an inner longitudinally-extending tube which is concentrically pinned to an outer longitudinally-extending tube which, in turn, is concentrically mounted within a heat transfer tube which radiates heat to the work.
Also, within the industrial burner art, there are numerous fuel fired burner arrangements which, at first glance, might bear some structural resemblance to the fuel fired radiant tube heater of the present invention, but which have entirely different functions and purposes associated with the structure.
Although more efficient than conventional "straight-through" radiant tube heaters, conventional recuperative-type radiant tube heaters possess certain limitations resulting from their design. For example, the service life of the inner tubes is limited due to the higher temperatures to which the inner tube is exposed relative to the outer tube. In addition, heat transfer from the combustion products to the outer radiant tube is restricted by the higher temperatures present within the inner tube relative to the outer tube.